250 
BULLETIN OF THE BUREAU OF FISHERIES. 
the pond begin to lower the fingerlings swarmed to the surface near the outlet. At 
the same time numbers of these water tigers appeared and attacked the fish from 
all sides. The dytiscid larva seized the fish by its throat and plunged its powerful 
mandibles into the flesh near the heart. After sucking the blood a moment it 
dropped the fish and attacked another, until the destruction became enormous. 
Similar attacks have been witnessed in the fishponds at Fairport, Iowa, but the 
number of fish killed was much less, owing to the smaller number of dytiscid larvae 
present. 
On May 18, 1916, 4,500 fry of Idiobus cyprinella, the common buffalofish, were 
placed in pond 14B, from which all the larger fish had been previously removed. 
From the time these fry were placed in the pond repeated search for them failed 
to reveal their presence. On July 12 the pond was drawn, but no buffalo fry were 
found. Many dytiscid larvae, however, appeared, and it was felt that they, with 
other enemies, might have contributed to the disappearance of the fish fry. No 
positive proof of this could be obtained, and since the dytiscid larva does not chew 
its prey but merely sucks its juices, it would seem as if there should have been some 
of the dead fry floating in the pond if the dytiscid larvae were really responsible for 
their disappearance. 
One of these attacks of a water tiger upon a small fish has been made 
the subject of a movie film. While this particular attack was undoubtedly 
staged for effect, it does reveal the voracity and tigerlike nature of the larva. 
There is no objection to artificial conditions when the reality has been repeatedly 
observed under perfectly natural conditions. Fortunately the larvae of other 
dytiscid genera are too small to function as enemies of fish. None of them, so 
far as known, have been reported as attacking young fish, and it is probable that 
ordinarily they are satisfied with other food. 
Of the Hydrophilidae the larvae of various species of the genus Hydrous have 
been captured while eating fish both in Europe and in America. The following 
is another instance to be added to a list that is already rather long. Ten thousand 
small fry of the buffalofish ( Idiobus cyprinella ) were placed in pond 3E, June 1, 1918. 
During the month many Hydrous larvae appeared in the pond and the buffalo fry 
began to disappear. By watching the pond closely the beetle larvae were actually 
captured while eating fish on July 3. The pond was then drained and the fry 
removed and about 150 full-grown Hydrous larvae were picked up, while many 
more larvae and pupae were obtained from pupal chambers around the pond shores. 
In this instance, therefore, it was actually proved that the beetle larvae were respon- 
sible for the disappearance of some of the fish. Of the remaining genera of the 
Hydrophilidae, Hydrophilus is about the only one whose larvae would be large 
enough to become a menace to small fish. It has not been reported as actually seen 
committing this depredation, but it might do so under stress of hunger. 
Of the Gyrinidae the larvae of Dineutes americanus have been seen eating small 
fish. In the early summer of 1916 the channel catfish ( Idalurus pundatus ) in 
pond 9D for the first time spawned and hatched a brood of young. During the 
subsequent lowering of the pond in order to secure and remove these fry, George 
Lay observed large numbers of Dineutes larvae attacking the young fish. Several 
